1. Raw materials

1.3. Yeast and sourdough

1.3.1 Yeast

What is yeast ?

Yeast is a micro-organism that is grown in yeast factories. Its scientific name is Saccharomyces cerevisiae. In this word you can see the Latin word "saccharo", which means sweet or sugar and the word "myces", which means, "mould". Baker's yeast is a unicellular mould that reproduces through a process, which is known as "budding".

We know that bakers use it to make the dough "rise"; without it, our bread would be like flat, hard cakes. In the days when people made their own bread, they would go to a brewer and get a jug of brewer's yeast. It was fluid and yellow. Nowadays, yeast is made commercially on a large scale. The yeast you buy at your market, the yellow lumps done up in paper, has been compressed for convenient handling.

Commercial yeast is a by-product of the whisky distillers. If you are a yeast producer your by-product will be methylated spirits.

Yeast is a plant, according to the biologists, and is capable of reproducing itself. A piece of yeast consists of minute cells, with walls composed of cellulose, and an interior of living matter called protoplasm. You can feed it with a solution of sugar to make it grow, or it can be "killed" by starvation or heat. The ancients did not use yeast as we know it today; they prepared a leaven or 'barm' (which has the same action) from ground millet kneaded with "must" out of wine-tubs.

Wheat bran was also used, kneaded with a three-days-old must, dried in the sun, and then made into little cakes. When required for making bread, the cakes were soaked in water, and then boiled with the finest flour, after which the whole was mixed in with the meal. Another old method for making barm was to prepare cakes of barley meal and water; these were baked on a hot hearth, or else in an earthen dish upon hot ashes and left until they turned reddish-brown. Afterwards, the cakes were kept shut up in a vessel until they turned quite sour. When wanted for leaven, they were first steeped in water. Two hundred twenty five grams of this was enough to make a quantity of bread of about 6,5 kg to rise.

General description of yeasts

Yeasts are not, like moulds, a clearly defined taxonomic group. They belong to the same club as the moulds. Yeasts are said to be unicellular fungi in contrast to moulds, which are multicellular.

Yeasts can be found in nature mainly in habitats, where substrates rich in sugars are present i.e. flower nectar, on all kind of fruits etc. They can cause spoilage of certain foodstuffs such as fruit juices, syrups, honey, meat, wine, beer, yoghurt etc. They also are used in a number of food preparation processes such as bread, beer, wine, vinegar, cheese and for the production of enzymes. Although there are pathogenous yeasts (such as Candida albicans) they are rarely found in foodstuffs.

Morphological characteristics.

Most of the time yeasts are ovoid but they also can be round, lemon or pear shaped, cylindrical or even be triangular. The size can vary quite a bit but in general yeasts are bigger then bacteria. The diameter varies between 1 and 5 µ, while the length might vary between 5 and 30 µ. Yeasts do not have flagellae.

Fermentation

The primary function of yeast is to supply carbon dioxide gas, which inflates the dough during proof and the early stages of baking (oven spring).

Carbon dioxide cannot form a gas bubble on its own it requires a "nucleating site" (i.e. somewhere it can gather to form a bubble). In fizzy drinks microscopic projections on the side of the bottle provide those sites, which is why when you release the pressure as you open the bottle you see "streams" of gas running from the sides. In bread dough the nucleating sites are provided by the nitrogen gas bubbles, trapped in the dough during mixing. The yeast has used up the oxygen from the air.

During proof stages the carbon dioxide goes into solution until the solution is saturated and then any more, which is generated, makes its way into the nitrogen gas bubbles, which grow in size, and the dough expands. The more yeast and the warmer the temperature the faster the expansion - we get oven spring because the maximum gassing rate occurs at 40 - 45°C.

In bulk fermentation stages we also get dough expansion from carbon dioxide generation but most of that is lost when the dough is knocked back and divided, so the yeast has to start over again.

Yeast also contributes to dough maturity/development. Though its role is minor compared to improvers in no-time doughs, it is more significant in bulk fermentation where the enzymes, especially the proteolytic ones (they modify the gluten proteins), play a significant role.

To sum up, then, the dough is aerated by the action of the yeast. The little cells we mentioned ferment the dough, and produce tiny bubbles of gas inside it. As a result, the dough gets fatter and bigger, and rises, of course. Thus when the dough is baked, you have a 'bold' loaf, light and airy; when you cut it you can see all the tiny holes formed by the gas, so that it looks like a sponge.

Yeasts constitute a group of single-celled (unicellular) fungi, a few species of which are commonly used to leaven bread and ferment alcoholic beverages. Most yeasts belong to the division Ascomycota. A few yeasts, such as Candida albicans, can cause infection in humans. More than one thousand species of yeasts have been described. The most commonly used yeast is Saccharomyces cerevisiae, which was domesticated for wine, bread, and beer production thousands of years ago.

Yeast physiology can be either obligate aerobic or facultative anaerobic. In the presence of oxygen, yeast will multiply through a mechanism known as budding. In the yeast factory this mechanism is used to grow yeast into a commercially available product called compressed yeast. Compressed yeast contains about 75 % water of which about 1/5 th sits between the yeast cells and not in the yeast cells. Depending on the moisture content and the size of the individual yeast cells, 1 gram of compressed yeast contains 8 to 13 billion yeast cells. In these circumstances, the yeast does not have access to nutrients and it must survive on the reserves stored in the cell. The yeast cell will slowly but surely use these reserves. As a result the cells grow weaker and weaker i.e. the yeast will lose its gassing power. This process is called "autolysis". The speed at which this process progresses, depends on the proteolytic activity of the cell. Proteases are enzymes that split proteins. The proteolytic activity depends on the temperature. The higher the temperature the faster the breakdown of proteins will take place. Therefore yeast should be stored in the fridge.

In the absence of oxygen, fermentative yeasts produce their energy by converting sugars into carbon dioxide and ethanol (alcohol). In brewing, the ethanol is bottled, while in baking the carbon dioxide raises the bread, and the ethanol evaporates. There is no known obligate anaerobic yeast.

An example with glucose as the substrate is

C 6H 12O 6 (glucose) ð 2C 2H 5OH + 2CO 2

Yeasts for leavening bread may be produced commercially or caught from the environment. Many types of yeast can be isolated from sugar-rich environmental samples. Some good examples include fruits and berries (such as grapes, apples or peaches), exudates from plants (such as plant saps or cacti). Some yeasts are found in association with insects.

The use of potatoes, water from potato boiling, eggs, or sugar in bread dough accelerates the growth of yeasts. Salt and fats such as butter slow yeast growth down. A common medium used for the cultivation of yeasts is called potato dextrose agar (PDA) or potato dextrose broth. Potato extract is made by autoclaving cut-up potatoes with water for 5 to 10 minutes and then decanting off the broth. Dextrose (glucose) is then added (10 g/L) and the medium is sterilized by autoclaving.

Yeast fermentations comprise the oldest and largest application of microbial technology. They are used for beer and wine fermentations and bread production.

There are many different kinds of yeast, which all belong to the Saccharomyces family. They don't have all the same capacity to ferment sugars. Certain species only ferment one type of sugar, others two different types of sugar. In the case of bread we need a yeast which can transform glucose and fructose into CO 2 and alcohol.

Yeast budding

Saccharomyces cerevisiae is also known as budding or baker's yeast. It is used as a model organism by biologists studying genetics and molecular biology (in particular the cell cycle) because it is easy to culture but as a eukaryote, it shares the complex internal cell structure of plants and animals.

Yeasts can reproduce asexually through budding or sexually through the formation of ascospores. A new organism is formed by the protrusion of part of another organism. When yeast buds, one cell becomes two cells. This is an example of reproduction. This is very common in plants, but may be found in animal organisms, such as the hydra, as well. Usually, the protrusion stays attached to the primary organism for a while, before becoming free. The new organism is naturally genetically identical to the primary one (a clone).

During asexual reproduction a new bud grows out of the parent yeast when the condition is right, then, after the bud reaches an adult size, it separates from the parent yeast. Under low nutrient conditions yeasts that are capable of sexual reproduction will form ascospores. Yeasts that are not capable of going through the full sexual cycle are classified in the genus Candida.

The cell cycle, or cell division cycle, is the cycle of events in an eukaryotic cell from one cell division to the next. It consists of interphase, mitosis, and usually cell division. The cell cycle is regulated by cyclins and cyclin-dependent kinases. Leland H. Hartwell, R. Timothy Hunt and Paul M. Nurse won the 2001 Nobel Prize in Physiology or Medicine for their discovery of these central molecules in the regulation of the cell cycle.

Schema of the cell cycle. I = interphase, M = mitosis.

The duration of mitosis in relation to the other phases has been exaggerated in this diagram

The phases of the cell cycle are:

• The G 0 phase is a period in the cell cycle where cells exist in a quiescent state.
• The G 1 phase is the first growth phase.
• S phase, during which the DNA is replicated (S stands for the synthesis of DNA)
• G 2 phase is the second growth phase, also the preparation phase for the cell.
• M phase or mitosis and cytokinesis, the actual division of the cell into two daughter cells.

A surveillance system, so-called "checkpoints", monitors the cell for DNA damage and failure to perform critical processes. Checkpoints can block progression through the phases of the cell cycle if certain conditions are not met. For instance, there is a checkpoint, which monitors DNA replication and keeps cells from proceeding to mitosis before DNA replication is completed. Similarly, the spindle checkpoint blocks the transition from metaphase to anaphase within mitosis if not all chromosomes are attached to the mitotic spindle.

Saccharomyces cerevisiae was the first eukaryotic genome that was completely sequenced. The yeast genome database is highly annotated and remains a very important tool for developing basic knowledge about the function and organization of eukaryotic cell genetics and physiology. The Munich Information Centre maintains another important S. cerevisiae database for Protein Sequences

Physiological characteristics

Most yeasts grow will in a liquid substrate or a substrate that is rich in water. Although yeasts require more free water than moulds, some of them grown pretty well in substrates containing a lot of dissolved substances such as sugar or salt. Where for "normal" yeasts the minimal a w value for growth lies between 0,88 and 0,94 (for instance 0,94 for yeast used in breweries, 0,905 for baker's yeast), osmophilic yeasts will also grow at a w values of 0,62 (for instance Zygosaccharomyces rouxii) to 0,65. This kind of values are found in syrups, jams, honey etc. Some osmophilic yeasts will not grow at a w values higher than 0,78. Obviously the optimum a w value as well as the a w range in which yeasts will grow depend also on other factors such as pH, temperature, oxygen, presence inhibitory substances etc.

Yeasts also have an optimum temperature at which they will grow. Normal values range between 25 and 30°C. The maximum temperatures yeasts can withstand are in the 35 – 47°C bracket. Some types grow at 0°C. Candida curiosa for instance grows between 0°C and 15°C but not anymore at 25°C.

An acid environment (pH 4,0 – 4,5) will favour the growth of yeast. Growth in an alkaline environment is feeble. Exceptions are Zygosaccharomyces balii which still grows at a pH value of 1,8 while other species such as certain Schizosaccharomyces types grow well at a pH > 7.

1.3.2. Sourdough

Introduction

How is it possible that out of four relatively tasteless or even bad tasting raw materials the baker can make such a fantastic product? In a poll, where people were asked to list according to their preference different smells, 71 % of all respondents put the smell of freshly baked bread on the first spot. The results were the following:

Definitions

Let's start by giving some definitions because there is quite a lot of confusion when it comes to sourdough. People also use rather carelessly various expressions, saying one thing but meaning another.

Sourdough is made from flour and water, which starts to ferment spontaneously and which is allowed to ferment for a certain time at a certain temperature. Indeed flour contains naturally lactic acid bacteria, which will develop in the mixture and which will acidify it. Sometimes the baker adds lactic acid bacteria himself. In French this is called a "levain", in Italian or Spanish "madre", in Dutch "zuurdesem" and the German baker will talk about "Sauerteig".

A sponge is made from flour, water and commercial available yeast. As a sourdough it is kept for a certain time at a certain temperature. In French this is called a "poolish", in Dutch a "zetsel". Italians will call it "biga" and the Germans use the word "Hefestück".

It seems appropriate to make a list of all terms used in the bakery and to give a definition of the various expressions.

Lactic acid bacteria

As said before, the term "lactic acid bacteria" is a collective noun and there are numerous species and subspecies of lactic acid bacteria. Lactobacillus literally means, "milk rod" because the take the shape of a little rod. They have very complex nutritional requirements for amino acids, peptides, vitamins, salts, fatty acids and fermentable sugars. The requirements are normally specific for each different kind of lactic acid bacteria. The temperature for optimal growth lies between 30°C and 40°C but they are active between 2°C and 53°C. Most lactic acid bacteria are micro-aerobe to anaerobe but most of them are aerotolerant. They survive well in acid conditions and the optimal pH lies between 5,5 and 6,2. They remain active at a pH as low as 3,5.

Lactic acid bacteria can be divided into two main families

• Obligatory homofermentative lactic acid bacteria : produce exclusively lactic acid out of maltose and glucose in anaerobic circumstances. As they do not produce CO 2, yeast has to be added to make the dough rise. The most common homofermentative lactic acid bacteria are Lactobacillus plantarum, Lactobacillus casei, Lactobacillus farciminis, Lactobacillus alimentarius and Lactobacillus acidophilus.

• Heterofermentative lactic acid bacteria (such as Lactobacillus sanfranciscensis) produce not only lactic acid but also ethanol and CO 2 using all kind of hexoses as a substrate (pentose phosphate cyclus). In the presence of certain ingredients (such as fructose, oxygen, citrates), not only ethanol will be produced but also acetic acid. As fructose is always present in dough but as it is not metabolised by lactic acid bacteria, a certain amount of acetic acid is always formed. The most important heterofermentative lactic acid bacteria are L. brevis lindneri, L. fermentum, L. sanfranciscensis, L. fructovorans and L. buchneri. Of all these types, L. brevis lindneri, L. fermetnum, L. plantarum en L. casei are important for flavour building during dough fermentation.

Both groups will contribute to the flavour profile of the bread. Important is to remember that lactic acid bacteria not necessarily impart a acid taste to the bread. The taste depends on the dominant group of lactic acid bacteria present in the dough. The total count of lactic acid bacteria in a sourdough is on average between the 2.10 8 and 6.10 9 cells per gram of sourdough.

The most important strains of lactic acid bacteria are :

Lactobacillus is a strain of rod shaped bacteria. They can be found in all kinds of vegetable origin but also in the intestines and the mouth. They are important in the production of yoghurt, sauerkraut, cottage cheese, beer, wine and obviously sourdough. In yoghurt one will find about 10 million lactobacilli in 1 gram of product.

Streptococci is a strain of sphere shaped bacteria and the name literally means "spheres which appear in strands". They can also be found in all kind of products and are amongst other things important during the production of sausages.

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Noël Haegens

info@classofoods.com

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